1. Field of the Invention
The present invention relates to an ohmic electrode, its fabricating method and a semiconductor device, and more particularly the present invention relates to an ohmic electrode for a III-V compound semiconductor, its fabricating method, and a semiconductor device using the ohmic electrode.
2. Description of the Related Arts
From the viewpoint of improving the performance and reliability of a device such as FET using a compound semiconductor, reducing the contact resistance of its ohmic electrode and improving its thermal stability are of great importance. However, with regards to compound semiconductors, particularly, III-V compound semiconductors such as GaAs semiconductors, there have been none that satisfy the above-indicated requirements.
Ohmic electrodes for GaAs semiconductors which have been put in practical use or proposed can generally be classified into the following three types. In ohmic electrodes of the first type, an ohmic metal used includes an element behaving as a donor impurity for GaAs semiconductors. By annealing, the element is diffused in the semiconductor thus making an n-type region having a high impurity concentration along the interface between the electrode metal and the semiconductor such that an ohmic contact is established due to a tunneling effect or the like. In ohmic electrodes of the second type, the ohmic electrode includes an element forming an intermediate layer of a low energy barrier. By annealing, the element makes an intermediate layer of a lower energy barrier between the electrode metal and the semiconductor such that an ohmic contact is established by lowering the energy barrier in the region where carriers move. In ohmic electrodes of the third type, the ohmic electrode includes an element wich reacts with GaAs semiconductors when annealed and makes a regrown semiconductor layer wich behaves as a donor impurity for GaAs semiconductors. By annealing, a regrown layer is formed and changes into an n-type having a high impurity concentration such that an ohmic contact is established due to a tunnelling effect or the like.
A typical example of ohmic electrodes of the first type is shown in FIGS. 1A and 1B. In this example, an AuGe/Ni film 2 as an ohmic metal is formed on an n.sup.+ -type GaAs substrate 1 as shown in FIG. 1A. Then, it is annealed at 400.about.500.degree. C. to make the ohmic electrode as shown in FIG. 1B. In FIG. 1B, reference numeral 3 denotes an n.sup.++ -type GaAs layer, and 4 denotes a layer including NiAs and .beta.-AuGa.
The ohmic electrode shown in FIG. 1B, however, has bad thermal stability. That is, in this case, a large amount of Au contained in the AuGe/Ni film 2 as the ohmic metal (typically used AuGe contains 88% Au) reacts with a n.sup.+ -type GaAs substrate 1 when annealed at a temperature of 400.degree. C. or higher and forms .beta.-AuGa (hexagonal close packed (HCP) structure, melting point T.sub.m =375.degree. C.) in the layer 4. Therefore, although the contact resistance of the ohmic electrode certainly decreases, the thermal stability deteriorates. As a result, the device characteristics deteriorate when a high temperature is applied in a process such as chemical vapor deposition (CVD) executed after the formation of the ohmic electrode. In addition, .beta.-AuGa produced by reaction of the n.sup.+ -type GaAs substrate 1 with Au in the AuGe/Ni film 2 causes a rough surface of the ohmic electrode and makes subsequent fine patterning difficult.
The ohmic electrode shown in FIG. 1B involves another problem that it cannot cope with thinning for the n.sup.++ -type Gas layer 3 and reduction of size of devices such as a. That is, since the n.sup.++ -type GaAs layer 3 is formed by diffusion during annealing, extension thereof in the depth direction and in the lateral direction (parallel to the substrate) is determined solely by the temperature and the time of the annealing. Therefore, extension of the n.sup.++ -type GaAs layer 3 in the depth and lateral directions cannot be controlled. As a result, it is difficult to reduce the thickness of the n.sup.++ -type GaAs layer 3 and the distances between ohmic electrodes for improving performance and reducing the size of device.
Ohmic electrodes of the second and third types have been proposed to overcome the problems caused by the use of the AuGe/Ni film 2 for fabricating an ohmic electrode in the typical example of the first type, i.e., the thermal instability of the ohmic electrode and a rough surface of the electrode.
A typical example of ohmic electrodes of the second type is shown in FIGS. 2A and 2B. In this example, sequentially provided on an n.sup.+ -type GaAs substrate 11 are a NiIn film 12 and a W film 13 as ohmic metals as shown in FIG. 2A. Thereafter, the structure is annealed for a second or so at a high temperature of about 900.degree. C. to form the ohmic electrode as shown in FIG. 2B. In FIG. 2B, reference numeral 14 denotes an InGaAs (abbreviated representation used hereinafter for In.sub.x Ga.sub.1-x As) layer, and 15 denotes a Ni.sub.3 In film. In this case, the InGaAs layer 14 as an intermediate layer of a low energy barrier is formed by reaction of the n.sup.+ -type GaAs substrate 11 and In in the NiIn film 12 by annealing, and an ohmic contact is established by a decrease in effective height of the energy barrier. Since the ohmic electrode shown in FIG. 2B does not include a compound such as .beta.-AuGa having a low melting point as included in the ohmic electrode of the first type shown in FIG. 1B, it has been reported that the contact resistance of the ohmic electrode is stable even with annealing at about 400.degree. C. for about 100 hours.
However, since the ohmic electrode shown in FIG. 2B requires annealing at a high temperature of about 900.degree. C. for establishing an ohmic contact, it cannot be used in devices such as a JFET (junction gate FET) and HEMT (high electron mobility transistor) in which a gate and a channel are formed at a temperature below 900.degree. C. Therefore, the ohmic electrode involves the problems that the process window is small and that its use is limited to only a few kinds of devices.
A typical example of ohmic electrodes of the third type is shown in FIG. 3. In this example, sequentially stacked on an n.sup.+ -type GaAs substrate 21 are a Pd film 22 and a Ge film 23 and ohmic metals as shown in FIG. 3A. The structure is annealed at a bout 325.about.375.degree. C. for about 30 minutes to form the ohmic electrode as shown in FIG. 3B. In FIG. 3B, reference numerals 24 denotes an n.sup.++ -type GaAs layer, and 25 denotes a PdGe film. In this case, while annealed, a regrown layer of GaAs is first formed from the n.sup.+ -type Gats substrate 21, and Ge in the Ge film 23 is then diffused into the regrown layer. As a result, the n.sup.++ -type GaAs layer 24 is formed to establish an ohmic contact.
With the ohmic electrode shown in FIG. 3B, the thickness of the regrown n.sup.++ -type GaAs layer 24 can be controlled by changing the thicknesses of the Pd film 22 and the Ge film 23 as ohmic metals. Therefore, the ohmic electrode of this type permits a decrease in thickness of the n.sup.++ -type GaAs layer 24 and in distances between ohmic electrodes. The ohmic electrode shown in FIG. 3B, however, has a serious problem in its thermal stability.
The above-discussed characteristics of the ohmic electrodes of the first, second and third types are summarized in Table 1.
TABLE 1 ______________________________________ difficulty short of contact thermal surface diffusion type process resistance stability flatness length ______________________________________ 1 good good bad bad bad 2 bad good good good good 3 good good bad good good present good good good good good invention ______________________________________
As mentioned above, since existing ohmic electrodes for GaAs semiconductors are unsatisfactory, realization of an ohmic electrode having practically satisfactory characteristics has been desired.